1. Field of the Invention
The present invention relates to a spatial light modulator (SLM) comprising a layer of photoconductive material associated with a layer of liquid crystal.
An SLM is generally constituted by a photoconductive layer juxtaposed with a layer of electro-optic material. The photosensitive layer detects an incident intensity distribution and induces a distribution of charges on the electro-optic material that locally modifies its optical properties. The light transmitted or reflected by an SLM thus undergoes a spatial modulation of its amplitude or its phase.
For, the addition of photons to a photoconductive material leads to a transfer of electrons from a fundamental energy level to an excited energy level, and hence to an increase in current that is expressed by a decrease in resistivity.
To carry out the selective transmission of information to the electro-optic material (liquid crystal (XL)), the photoconductor (PC]should be highly resistive in darkness and should meet the following condition: EQU (.rho..sub.PC) illumination &lt;&lt;.rho..sub.XL.
where .rho. represents the resistivity.
The voltage applied to the liquid crystal is then:
negligible in darkness;
substantially equal to the voltage applied to both layers in their entirety for an incident flux that is sufficient and hence capable of modifying the optic properties of the liquid crystal (i.e. capable of varying the optic index of the birefringent material) in case of illumination.
SLMs are used in transmission in the optic processing of signals. For example they are used in incoherent-coherent conversion, wavelength conversion or optic amplification (cf. R.S. Mc Euen, J. Phys. E. Scien. Instr. 20, 1987). They can be used in reflection in the projection of cathode-ray tube (CRT) images on wide screens, more generally known as projection television. The association of three CRTs and three SLMs enables the modulation of the three colors, red, green and blue, and thus enables the recomposition, by mixing, of a color television image projected on a wide screen (cf. R.J. Fergenblatt, Seminar 14, Electronic Projection Displays, 1987).
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2. Description of the Prior Art
At present, in SLMs used in transmission and in reflection, the photoconductive layer is made of inorganic materials such as:
intrinsic or doped photoconductive semiconductors: these are semiconductors wherein the forbidden band is narrow enough to transfer electrons from the valence band to the conduction band under the effect of photons. In this case, their sensitivity is restricted to the visible and near infrared portions of the spectrum. To extend this sensitivity up to the far infrared portion, the semiconductors may be doped with a metal. For, doping enables the introduction of additional energy levels that facilitate excitation by low-energy photons. For example, SLMs have been made out of CdS (cf. Grinberg et al., Opt. Engin. 14 217, 1975) or amorphous silicon a-Si (cf. D. Williams et al , J. Phys. D. Appl. Phys. 21, 1988).
crystals ever since the photoconductive
properties of Bi.sub.12 SiO.sub.20 were revealed (cf. Aubourg et al., Appl. Opt. 21 3706-12, 1982), SLMs formed by this crystal have shown very promising performance characteristics.
Concomitantly with all these inorganic compounds, Mylnivok et al. (Sov. Tech. Phys. Lett. 11 (1) and 30 (4) 1985 and 32 (10) 1987), have attempted to introduce polymer materials as photoconductors in this type of application. They have used a polyimide that has promising mechanical and thermal properties and can be doped to give a photoconductive material. This polyimide does not have a photoconductive effect per se.
As a general rule, the performance characteristics of an SLM can be assessed according to the following criteria:
high mobility of the carriers (.mu.)
high resisitivity of the photoconductor in the dark (.rho..sub.o);
high sensitivity to the light flux (.beta.). The conductivity is related to the light flux by the following equation: .sigma.=.sigma..sub.o +.beta.I
I: intensity of the light flux;
.beta.: depends on .mu., .tau. (lifetime), .eta. (quantum yield)
.rho.: 1/.rho..sub.o conductivity in the dark
high spatial resolution.
A high mobility of the carriers makes it possible to obtain a fast response time, specifically a fast time of extinction to 10%. This response time should be of the same order of magnitude as that of the response of the liquid crystal (about 100 .mu.s to 1 ms). The sensitivity, namely the power per unit of area needed to go from 10% to 90% of transmission enabling high efficiency to be achieved has to be located between 1 and 3 mW/cm.sup.2.
The spatial resolution related to the nature of the photoconductor as well as to its dimensions is presently 70 pairs of lines (pl)/mm (in the case of a-Si), 40 pl/mm for CdS and 12 pl/mm for Bi.sub.12 SiO.sub.20 (the thickness of which, for technological reasons, can be brought to 300.mu.m with difficulty).